The present invention relates to emulsions containing alcohol and/or water in a dispersed phase and a hydrocarbon liquid in the continuous phase. More particularly the present invention relates to such emulsions that are resistant to stratification on relatively long term undisturbed storage at ambient temperature.
It is known from the applicant""s Australian patent specification 544.728 that emulsions containing alcohol and/or water in one phase and a hydrocarbon liquid in the other may be formed using an emulsifier that is a block copolymer of ethylene oxide and styrene type monomers. These emulsions are of particular utility as liquid transport fuel in which hydrated methanol or hydrated ethanol is dispersed in diesel oil or gasoline. Hydrated alcohol in diesel oil (termed AW/D) emulsions represent a new and potentially cost effective option for significantly reducing particulate and NOx emissions from diesel engines. Emulsions of hydrated alcohol in gasoline exhibit reduced vapour pressure, increased water tolerance and reduced NOx emission on combustion as compared with anhydrous alcohol solutions in gasoline. When the alcohol is ethanol produced from biomass there can be a reduction in the net emission of carbon dioxide (xe2x80x9cgreenhouse gasxe2x80x9d) as compared with the use of unblended petroleum fuel.
The arrangement described in the above mentioned specification utilised not only a block copolymer as the emulsifier but also a block copolymer as a coupling agent. The coupling agent typically comprised a block copolymer of butadiene and styrene type monomers and was described as containing butadiene and styrene in weight ratios of at least 2:1, preferably at least 3:1. The emulsifier and the coupling agent are described in that specification as being present in the emulsion in a ratio of from 1:3 to 3:1. In these ratios solutions of the two copolymers are compatible.
It has been found that while the AW/D emulsions described in the aforementioned Australian patent specification produce generally stable emulsions they do suffer from a problem of stratification, i.e., a tendency for the dispersed or discontinuous phase droplets to rise if they are less dense than the continuous phase of the emulsion, called xe2x80x98creamingxe2x80x99, or to fall if they are more dense than the continuous phase, called xe2x80x98settlingxe2x80x99. While this stratification may take a relatively long time and may be readily reversed by gentle agitation it is a major commercial disadvantage as there is the possibility that any given aliquot of emulsion may, after a period of relatively long-term undisturbed storage, contain more or less of the dispersed phase component than is intended. In the case of an emulsion used as a fuel such stratification makes the starting and/or running of an engine difficult, if not impossible.
The present invention relates to emulsions containing in a continuous phase a hydrocarbon liquid and in a dispersed phase alcohol and/or water in the presence of:
(a) an emulsifier being a copolymer containing at least one relatively hydrophobic polymeric block of at least one monomer selected from the group comprising styrene and ring substituted styrenes, and at least one relatively hydrophilic polymeric block of at least one monomer selected from the group having the formula H(Oxe2x80x94R)n OH where R is an aliphatic radical containing from 1 to 4 carbon atoms and n is a number between 20 and 300, and
(b) a coupling agent being a copolymer containing at least one polymeric block of at least one monomer selected from the group comprising styrene and ring substituted styrenes, and at least one block being a saturated or unsaturated aliphatic hydrocarbon moiety, the emulsifier being present in an amount sufficient to form micelles of the relatively hydrophilic polymer block sufficient to contain the dispersed phase and the coupling agent being present in an amount that is sufficient to substantially prevent the dispersed phase droplets from coalescing while not being present in a sufficiently high amount that micelles of the styrene containing polymeric blocks of the coupling agent form in the continuous phase of the emulsion.
The absolute amounts of the emulsifier and the coupling agent can be determined for any given emulsifier and coupling agent by quantitative testing. It is apparent to the present inventor that the amount of the coupling agent in the emulsion is important and typically approaches the solubility limit of the coupling agent in the continuous phase. If the amount of the coupling agent is lower than the critical amount it will not couple effectively with the micelles of the emulsifier and therefore stratification of the dispersed phase of the emulsion will not be inhibited. If the amount of the coupling agent is higher than the critical amount it will form micelles of the styrene containing polymeric blocks of the coupling agent which will in turn cause flocculation of the dispersed phase to occur. The solubility of the coupling agent in the continuous phase can be adjusted by an alteration of the amount of the styrene moiety relative to the hydrocarbon moiety in the coupling agent. The solubility will also depend to some extent upon the aromaticity of the hydrocarbon liquid comprising the continuous phase. The more aromatic is the hydrocarbon liquid the higher will be the solubility of any given coupling agent.
The maximum amount of emulsifier is not as critical as the maximum amount of the coupling agent as, within reasonable limits, the presence of an excess of emulsifier has only an economic disadvantage and not a technical one. The emulsifier should be present in an amount greater than 3 times the amount of the coupling agent as is described in more detail later in this specification. Preferred amounts of the coupling agent in the emulsion are in the range of from 0.75 to 1.5 gm/l. When the dispersed phase is hydrated methanol it is preferred that the coupling agent is present in the emulsion in the range of from 0.75 to 1.0 gm/l whereas in the case of the dispersed phase comprising hydrated ethanol the preferred range is from 1.0 to 1.5 gm/l.
In total the copolymer content of the emulsion, i.e. the combined amount of the emulsifier and of the coupling agent, is preferably in the range of from 4 to 10 gm/l. In the case of emulsions containing hydrated methanol in an amount of 12% by volume in the dispersed phase the preferred amount of copolymers is 4 to 6 gm/l, more preferably 5 gm/l, and in the case of emulsions containing hydrated ethanol in an amount of 15% by volume in the dispersed phase the preferred concentration of copolymers is 5 to 8 gm/l, more preferably 6 gm/l. In the case of emulsions that contain greater amounts of alcohol and/or water in the dispersed phase the preferred amount of the copolymers will typically remain less than 10 gm/l, however in this case the ratio of the emulsifier to the coupling agent will rise. This is because, as the amount of the dispersed phase increases so the quantity of emulsifier required increases in proportion to the increase in the surface area of the dispersed phase of the emulsion; by contrast, the quantity of coupling agent stays substantially constant as the amount of the dispersed phase increases.
As used in this specification the following terms have the meanings indicated:
The hydrocarbon liquid is preferably selected from the group comprising gasolines, kerosenes, diesel oils, gas oils and fuel oils. The gasolines are liquid hydrocarbons boiling below 190xc2x0 C. The kerosenes, diesel oils and gas oils boil between 170xc2x0 and 480xc2x0 C., while the fuel oils boil above 450xc2x0 C.
The hydrocarbon liquid is preferably selected from the gasolines, the kerosenes which boil between 170 and 230xc2x0 C., the diesel oils which boil from 170 to 315xc2x0 C. and the gas oils that boil from 315 to 480xc2x0 C. The invention is particularly applicable to the high boiling point kerosenes, the diesel oils and the low boiling point gas oils when the dispersed phase comprises a hydrated alcohol. In the case where the dispersed phase comprises only water the preferred hydrocarbon liquid is selected from the diesel oils, the gas oils, the fuel oils and the hydrocarbon liquids boiling above 480xc2x0 C.
The dispersed phase of the emulsions preferably comprises a hydrated alcohol, which preferably contains from 0.1 to 10% by volume of water. The dispersed phase is preferably methanol or ethanol, or mixtures thereof, together with water.
The proportions of alcohol and/or water to hydrocarbon liquid may vary over wide limits. For alcohol containing emulsions, a hydrated alcohol content of between 10 and 30% by volume is preferred.
The emulsifier is preferably a copolymer containing at least one polymeric block of at least one monomer selected from the group comprising styrene and ring substituted styrenes and at least one polymeric block of at least one monomer selected from the group having the formula H(Oxe2x80x94R)n OH, where R is an aliphatic bivalent hydrocarbon radical containing from 1 to 4 carbon atoms and n is a number between 20 and 300, more preferably between 22 and 130. The emulsifier is preferably a pure di-block copolymer or a reaction mixture containing principally di-block copolymer and tri-block copolymer having two blocks of the styrene monomer, together with some unreacted homopolymer of the H(Oxe2x80x94R)n OH monomer.
In a particularly preferred embodiment of the present invention the emulsifier is a polyethylene oxide-polystyrene copolymer. The polyethylene oxide block is preferably formed first with a molecular weight of between 1,000 and 10,000. The polyethylene oxide and the polystyrene are preferably present in a weight ratio of 1 part of polyethylene oxide to from 0.8 to 1.5 parts of polystyrene, more preferably from 1.0 to 1.25 parts of polystyrene. In preferred embodiments of the invention the emulsifier is produced by the reaction of styrene monomer with polyethylene oxide homopolymer in the presence of a free radical initiator. The reaction product formed without chain transfer agents, comprising polyethylene oxide-polystyrene di-block copolymer . . . polystyrene-polyethylene oxide-polystyrene tri-block copolymer, unreacted polyethylene oxide homopolymer and a minor proportion of various other higher molecular weight dimeric and trimeric species and polystyrene homopolymer, is particularly suitable for use in the present invention.
The coupling agent is preferably a copolymer containing at least one polymeric block of at least one monomer selected from the group comprising styrene and ring substituted styrenes and at least one polymeric block of at least one compound selected from the group comprising saturated or unsaturated aliphatic hydrocarbons, preferably butadiene or isoprene. Alternatively the hydrocarbon may comprise a single long chain hydrocarbon moiety rather than a polymer made up of a plurality of recurring monomeric entities. The coupling agent is preferably a pure diblock copolymer or a tapered block copolymer. Such copolymers are preferably produced by anionic solution polymerisation as this technique produces copolymers with a defined structure and a narrow molecular weight distribution. It is preferred that the coupling agent contains styrene and the aliphatic hydrocarbon in the weight ratio of from 1:0.3 to 1:1, most preferably from 1:0.42 to 1:0.66. These ratios are outside the range of at least 1:2 and most preferably 1:3 stated to be desirable in Australian patent specification 544.728 and lead in an opposite direction from the teaching of that specification.
The coupling agent preferably has a molecular weight of from 13,000 to 400,000, more preferably the 100,000 to 170,000, most preferably 120,000 to 150,000. It is preferred that styrene comprises from 50 to 77% by weight of the coupling agent, most preferably 50 to 70% depending on the aromatic content of the continuous phase of the emulsion. It has been found that when the aromatic content of the continuous phase is less than 22% it is preferable for the styrene content of the coupling agent to be in the range of from 50 to 60% by weight. By contrast when the aromatic content of the continuous phase is more than 23% it is preferable for the styrene content of the coupling agent to be in the range of from 60 to 70% by weight.
As has been previously described, at the preferred proportions of styrene the solubility of the coupling agent is limited in the continuous phase of the emulsion. This limited solubility encourages coupling of the styrene moieties on the emulsifier with those of the coupling agent thus preventing coalescence of the droplets in the dispersed phase of the emulsion.
It has surprisingly been found that the emulsifier and the coupling agent are desirably included in the emulsion in quite specific amounts. The emulsifier should preferably be present in such an amount that the emulsifier is able to form micelles of its relatively hydrophilic polymer (typically polyethylene oxide) block(s) in a concentration sufficient to contain the alcohol and/or water dispersed phase. The coupling agent should be present in an amount that enables it to couple with, or to form polystyrene micelles only with, the styrene moieties on the emulsifier and to thereby substantially prevent the dispersed phase droplets from coalescing, while not being present in an amount sufficient to form polystyrene micelles of the styrene containing polymeric blocks of the coupling agent itself in the continuous phase of the emulsion. This means that in preferred embodiments of the invention the emulsifier and the coupling agent will be present in a weight ratio of more than 3.0:1, preferably at least 3.5:1, and more preferably at least 4:1. It is surprising that, at these preferred ratios, solutions of the emulsifier and of the coupling agent are not compatible. These ratios are outside the range of 1:3 to 3:1 stated to be desirable in Australian patent specification 544.728 and lead in an opposite direction from the teaching of that specification.
Without being limited to the theory it is suggested that the mechanism of the present emulsification is one of steric stabilisation. It is considered that the emulsifier will form hydrophilic (typically polyethylene oxide) micelles in the continuous phase of the emulsion. The alcohol and/or water is contained within these micelles, the quantity of the alcohol and/or water that can be accommodated in the emulsion is therefor dependent upon the concentration of these micelles in the emulsion. The styrene block or blocks of the emulsifier have a limited solubility in the continuous phase but will project into the continuous phase from the micelles as their solubility is higher in the continuous phase than in the discontinuous or dispersed phase.
The coupling agent has blocks of styrene containing polymer which are only of limited solubility in the continuous phase and a hydrocarbon moiety which is of much greater solubility in the continuous phase of the emulsion. It is believed that the styrene containing polymer blocks of the coupling agent will couple with the corresponding styrene containing polymer blocks of the emulsifier to form a zone of coupling agent surrounding each micelle: If, however, the amount of coupling agent is increased above a certain concentration, then, as has been discovered by the present inventor, in addition to coupling with the emulsifier the coupling agent molecules will couple with themselves to form polystyrene micelles. If this happens then rather than acting to maintain a sub-micron dispersion of alcohol and/or water droplets the coupling agent unexpectedly acts to flocculate the dispersed phase, forming comparatively large sized droplets. The presence of the coupling agent is however essential to the maintenance of the sub-micron size dispersion of alcohol and/or water droplets. In the absence of the coupling agent the micelles of emulsifier flocculate, as the styrene containing polymer blocks of the emulsifier in adjacent micelles couple together.
The dispersed phase droplets behave in accordance with Stokes Law and may stratify in the emulsion in accordance with the number of droplets, the droplet diameter, the relative density of the dispersed and continuous phases of the emulsion and the viscosity of the continuous phase. The stratification, which is manifest either as creaming, where the dispersed phase rises in the continuous phase, or settling, where the dispersed phase sinks in the continuous phase, may be prevented, or at least reduced, by the establishment of a suitable sub-micron size distribution of the dispersed phase droplets.
It is believed that the emulsifier will form hydrophilic (typically polyethylene oxide) micelles in the continuous phase of the emulsion and, provided the concentration of these micelles is above the necessary minimum concentration for the volume of alcohol and/or water to be incorporated into the emulsion, will contain the alcohol and/or water inside the micelles to form the dispersed phase. It is believed that these initially formed dispersed phase droplets are of a sufficiently small size that they are not susceptible to stratification. It is further believed that if that these initially formed dispersed phase droplets can be prevented from coalescing or agglomerating the emulsion will remain resistant to stratification. If the coupling agent is present in a sufficient amount to couple with the styrene polymer blocks of the emulsifier, but without the formation of micelles of its own, then the initially formed dispersed phase droplets will be held apart and will not be able to couple with one another and coalesce the dispersed phase droplets. The micelles of the emulsifier will thus be stabilised by coupling agent molecules with their styrene polymer blocks coupled to the emulsifier and their hydrocarbon blocks solvated by the continuous phase of the emulsion.
While the correct selection of the emulsifier and the coupling agent and their relative amounts are important the present inventor has found that a preferred process of forming the emulsion is also desirable. The emulsifier is preferably first dissolved in the alcohol and/or water. In order to induce dissolution of the emulsifier it may be necessary to adjust the reaction conditions used to form the emulsifier to render it more hydrophilic and/or to use an aqueous solution of an alcohol to dissolve the emulsifier. The coupling agent is preferably first dissolved in the hydrocarbon liquid. The alcohol and/or water containing the dissolved emulsifier is then added to the hydrocarbon liquid containing the dissolved coupling agent. It is preferred that the solution of the coupling agent in the hydrocarbon liquid be subjected to agitation both before and after the addition of the alcohol and/or water containing the dissolved emulsifier.
The agitation of the components to the emulsion is preferably carried out using static in-line mixers. In the case of the agitation following the addition of the alcohol and/or water containing the emulsifier to the hydrocarbon liquid containing the dissolved coupling agent it is desirable that the mixing be carried out with high shear. In preferred embodiments of the invention the agitation, combined with the selected emulsifier and coupling agent, and the use of the process according to the present invention produces emulsions with dispersed droplets having diameters in the range of from 0.1 to 2.0 microns, more preferably from 0.1 to 0.9 microns with an average diameter of about 0.5 microns.